Update testing and development requirements
Update to latest Paypal API Version: 119
Use django-oscar `get_model`
Fix Paypal response for Python3.x

The mocked `response.content` was returning a str object in python2.7 and a unicode string in python3.x, but it should return  a byte string.
Tests were passing even if byte stings were not handled correctly in python3.

Use `response.text` instead of `response.content` (it returns an unicode object).
Use unicode string when `requests` app is involved with `from __future__ import unicode_literals`
Merge pull request #87 from itbabu/fix-python3-response

Fix handling of Paypal response under Python3